A microbial fuel cell is a device to convert, into electrical energy, chemical energy of organic matters included in waste water by catalytic action (metabolic reaction or biochemical transformation) of microbes, and further subject the organic matters to oxidative decomposition treatment.
For example, a document “JP 2009-93861 A” discloses a microbial fuel cell configured as follow: a negative electrode, to which anaerobic microbes are adhered, is held in a sealed internal space of an electrolytic cell, while an organic substrate is retained and the negative electrode is immersed; an ion-transfer-layer cassette is inserted in the internal space of this electrolytic cell; at least part of an outer shell of the cassette includes an ion permeable transfer layer; and a positive electrode is enclosed or coupled inside the cassette. In this microbial fuel cell, two kinds of vent pipes are further connected to the ion-transfer-layer cassette: a vent pipe for supplying gas into the ion-transfer-layer cassette; and a vent pipe for exhausting gas from the inside of the ion-transfer-layer cassette. The vent pipes are pulled out of the electrolytic cell from the ion-transfer-layer cassette. Oxygen that is fuel is supplied into the ion-transfer-layer cassette through one of the vent pipes.
However, in case oxidizing gas such as the oxygen is supplied to the microbial fuel cell, a complicated design for piping is needed, when the two kinds of vent pipes, namely, the vent pipe for supplying and the vent pipe for exhausting, are pulled out of the electrolytic cell from the inside thereof, as the technique disclosed in the above document: JP 2009-93861 A. In addition, because the cost required for supplying the oxygen to the microbial fuel cell is high, the restriction on the use is increased. On the other hand, if the oxygen is supplied to the microbial fuel cell by only natural diffusion of air, it is difficult to supply a sufficient amount of the oxygen to the microbial fuel cell, particularly, in case the size of the microbial fuel cell is large.